He fought with distinction in Prussia during the last three years of the Polish War (1626–1629) and also, as colonel of a regiment of horse, in 1630 in Germany. After the death of Gustavus Adolphus in 1632 his military yielded to his political activity. He had been elected president or Lantmarskalk in the Riksdag of 1629, and in the following year was created a Privy Councillor. In 1635 he conducted the negotiations for an armistice with Poland[1] (Treaty of Stuhmsdorf).

In 1637–1640 and again in 1648–1654 he was Governor General in Finland, to which country he rendered inestimable services by his wise and provident rule. He reformed the whole administration, introduced a postal system, founded ten new towns, improved and developed commerce and agriculture, and very greatly promoted education.[1] In 1640 he opened the Royal Academy of Turku, of which he was the founder, and first chancellor.[1] Still today the expression "Kreivin aikaan", or "at Count's Time" in the Finnish language means "at the correct/good time". The base of his statue in Turku bears the inscription "I was well pleased with the land and the land with me".

After the death of King Charles X in 1660, Brahe, as Lord High Steward, became one of the regents of Sweden[2] for the second time (he had held a similar office during the minority of Queen Christina, 1632–1644), and during the difficult year 1660 he had entire control of both foreign and domestic affairs.[1] He died on 2 September 1680 at his castle Bogesund in Uppland. He also held the castles Visingsborg at Visingsö and Brahehus on the mainland by Gränna, where during his lifetime he had held more than regal pomp.[1]